Long term evolution (“LTE”) of the Third Generation Partnership Project (“3GPP”), also referred to as 3GPP LTE, refers to research and development involving the 3GPP Release 8 and beyond, which is the name generally used to describe an ongoing effort across the industry aimed at identifying technologies and capabilities that can improve systems such as the universal mobile telecommunication system (“UMTS”). The goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards. The 3GPP LTE project is not itself a standard-generating effort, but will result in new recommendations for standards for the UMTS.
The Evolved Universal Terrestrial Radio Access Network (“E-UTRAN”) in 3GPP includes base stations providing user plane (including packet data convergence protocol/radio link control/medium access control/physical (“PDCP/RLC/MAC/PHY”) sublayers) and control plane (including radio resource control (“RRC”) sublayer) protocol terminations towards wireless communication devices such as cellular telephones. A wireless communication device or terminal is generally known as user equipment (“UE”). A base station (“BS”) is an entity of a communication network often referred to as a Node B or an NB. Particularly in the E-UTRAN, an “evolved” base station is referred to as an eNodeB. For details about the overall architecture of the E-UTRAN, see 3GPP Technical Specification (“TS”) 36.300 v8.5.0 (2008-05), which is incorporated herein by reference.
As wireless communication systems such as cellular telephone, satellite, and microwave communication systems become widely deployed and continue to attract a growing number of users, there is a pressing need to accommodate a large and variable number of communication devices transmitting a growing range of communication applications with fixed resources. Traditional communication system designs employing a fixed communication resource have become challenged to accommodate the rapidly growing customer base and the expanding levels of service. One area that has challenged the need to expand communication links over longer distances is the use of a communication node such as an intermediate relay node between user equipment and a base station. Relaying by an intermediate node between a user equipment and a base station can be employed in wireless communication systems to increase system radio coverage area, enhance channel reliability, provide cooperative diversity, offer inter-user cooperation, etc.
In relay-enhanced wireless communication systems, an access communication link or access link (a source-to-relay node link) and a relay communication link or relay link (a relay-to-destination node link) may provide less than desirable performance and different levels of communication channel quality in different radio environments, particularly in the presence of various levels of noise in a communication link. To improve the overall throughput of a wireless communication system, an adaptive modulation and coding (“AMC”) strategy may be applied separately to the access link and the relay link. For example, for the uplink case from user equipment to a base station through a relay node, the access link is from the user equipment to the relay node, which may be the less reliable link. The relay link refers to the link from a relay node to a base station, and this link may be the more reliable link. In this case, the modulation order of the relay link may be higher than that of the access link.
To include more general, practical communication systems, the access and relay links may be referred to as a “first hop” and a “second hop.” In the uplink case, the first hop refers to a link from the user equipment to a relay node, and the second hop refers to the link from the relay node to a base station or to a relay node to another relay node. In this general description, in a downlink case of a one relay-enhanced wireless communication system, the first hop may refer to the link from a base station to a relay node, and the second hop may refer to a link from a relay node to a user equipment.
To provide reliable and efficient communication from user equipment through a relay node to a base station, it is necessary to re-modulate and forward received signals at a relay node operating under a demodulate and forward (“DmF”) mode, as described in IEEE Standard C802, 16j-07/052r13, entitled “Demodulation and Forwarding Method in Relay Stations,” Jul. 19, 2007, which is incorporated herein by reference. A demodulate and forward relay node demodulates and makes hard detection decisions on its received signals to form an estimated codeword, and then forwards the estimated codeword to the destination node.
In a conventional wireless communication system employing a relay node, the bits of a received signal are sequentially mapped into the bits of a transmitted signal. In view of the growing deployment of communication systems such as cellular communication systems and market expectations for reliable communication between a base station and the user equipment, it would be beneficial to incorporate improvements for mapping the bits of a received signal at a relay node on to a transmitted signal when the transmitted signal is transmitted with a modulation scheme different from the modulation scheme of the received signal. Therefore, what is needed in the art is a system and method that avoids deficiencies of bit mapping schemes at a relay node employed in present communication systems.